FIG. 5 illustrates a structure of a conventional general CIS based thin-film solar cell. A CIS based thin-film solar cell 20 has a structure obtained by sequentially stacking a metal back electrode layer 2, a p-type CIS based light absorbing layer 13, an n-type high resistance buffer layer 4 and an n-type transparent conductive film window layer 5 on a glass substrate 1. Herein, the p-type CIS based light absorbing layer 13 is composed of a p-type I-III-VI2 group chalcopyrite semiconductor such as CuInSe2(CIS), CuInS2(CIS), CuIn(SSe)2(CISS), CuGaSe2(CGS), CuGaS2(CGS), CuGa(SSe)2(CGSS), Cu(InGa)Se2(CIGS), Cu(InGa)S2(CIGS), Cu(InGa) (SSe)2(CIGSS) or a CIS based compound semiconductor thin-film such as CIGS having a thin-film layer of CIGSS or the like as a surface layer.
In order to produce the p-type CIS based light absorbing layer 13 by a selenization/sulfurization method, selenization and/or sulfurization of a metal precursor film (hereinafter, also referred to as a film-forming object) having a stacked structure formed of any one of Cu/Ga, Cu/In and Cu—Ga/In is performed on the metal back electrode layer 2 on the glass substrate 1.
Selenization is performed by installing the film-forming object in a device and replacing an inside of the device with inert gas such as nitrogen gas, and thereafter introducing a selenium source and increasing a temperature to hold the object for a certain time at a certain temperature.
Also, sulfurization is performed by installing the film-forming object in the device and replacing the inside of the device with an inert gas such as nitrogen gas, and thereafter, introducing a sulfur source such as sulfurized gas, and increasing the temperature to hold the object for a certain time at a certain temperature.
At the time of selenization and sulfurization, the inside of the device is first replaced with an inert gas such as nitrogen gas, thereafter the selenium source is introduced, and as illustrated in FIG. 6, the temperature is increased and the object is held for a certain time at a certain temperature for selenization 31, then, in a state in which selenium atmosphere in the device is replaced with sulfur atmosphere, the temperature in the device is further increased and sulfurization 32 is performed by holding the object for a certain time at a certain temperature, thereby forming the sulfide/selenide-based CIS based light absorbing layer 13 (refer to a temperature profile illustrated in FIG. 6).
The CIS based compound semiconductor thin-film solar cell has a tendency to increase contents of Ga and S, which are composition components of the light absorbing layer 13, to improve the conversion efficiency (refer to Patent Documents 1, 2 and 3, for example). Patent Document 1 discloses that the high conversion efficiency may be obtained by setting the Ga content in a range not smaller than 0.117 and not larger than 0.434 (Ga composition in a III-group element) by a multi source coevaporation method when forming the light absorbing layer in the CIS based compound semiconductor thin-film solar cell.
Patent Document 1: Japanese Patent Application Laid-Open No. 9-829992
Patent Document 2: Japanese Patent Application Laid-Open No. 10-135495
Patent Document 3: Japanese Patent Application Laid-Open No. 10-135498